Double Whip Rigging and forces on Spar

[Serf Life]

I'm hearing Scotty

 

[Evo]

I'd like to make a small correction. The two short dwt rope legs aren't the same as SRT base tied tip (upside down), they're the same as DRT. The two tensions don't add up to less than the log weight, they add up to the log weight. As the dwt short leg from the log's pullley/xring to the dwt tree tip pulley/xring tension goes down, the short rope leg from the log's pullley/xring to the fixed dwt tree tip tension goes equally up.

The sentences:
So at the rig tip a) has log weight + bit less than log weight in down leg. b) has 1/2 log + bit less than 1/2 log + bit less than ( a bit less than) 50% log weight in down leg. So roughly 2x log vs 1 1/2x log - a clear win for dwt.

should be:
So at the rig tip a) has log weight + bit less than log weight in down leg. b) has 1/2 log +(change in short leg tension) + 1/2 log -(change in short leg tension) + bit less than ( a bit less than) 50% log weight in down leg. So roughly 2x log vs 1 1/2x log - a clear win for dwt.


Sorry. I'll drink more coffee.

I'd like to emphasize that the down leg rope tension can be dramatically reduced by using rings or biners, but on a conifer the rig tips are right on the stem so there's very little stem bending torque created vs a deciduous lateral branch rig tip. I'd say the sledge hammering of a crispy spar would be a concern right up there with let it run.

Hope it goes well.

Yup thanks for the correction. Unlike a upside down climber, one can cut smaller to get the same effect with a conventional method.

If the OP @NWPEZ wants some added security consider a Yates Screamer or the equivalent built in to the rigging system.

 

[Bart_]

A more detailed shock load force analysis for locked-catching a traditional -ve rigged spar chunk is in the HSE report. Pretty sure search will root it out. I test rig confirmed their results. For practical reasons I don't think anyone has ever quantified the sledge hammer effect. How could you measure it?!

A bunch of intermediary posts just showed up for me. The same sledge hammer occurs for a locked -ve spar chunk traditional or dwt because the log chunk follows the same trajectory. The rig tip load is reduced simplistically x2 vs x1.5 dwt and the dwt rope tensions are different with mainly the down leg being about half as big as traditional -ve rig.

The big driver for the sledge hammer effect is a roughly fixed portion of the falling log's kinetic energy being direction-changed from falling trajectory direction to arcing inwards toward the stem - that's actually the source of the big 5G catch force spike. So if you let it run you take away a lot of this gnarly direction change force spike and also the sledging velocity ie smaller sledge hammer blow to the trunk. With let it run it's more of a glancing blow to the trunk. I would even say an expert groundie could have the chunk contacting the ground before the chunk swings back to the spar ie not even a glancing blow.

When I used the phrase a bit less than, you can substitute in the actual rope tension change by using the appropriate tension ratio for the device you use. For a block, 1/1.2 = 0.83 tension, for an xring I think 1/1.4 = 0.71 tension and IIRC a biner 1/1.6 = 0.62 tension. I didn't want to muddy the waters for those who haven't familiarised themselves with the tension ratio principle yet.

To reiterate, the more rope in the system is almost negligible in the case of traditional -ve rig vs dwt for a hard stopped pre-locked spar chunk. The rig tip and hence spar loading is definitely less in this case for dwt vs traditional. x1 1/2 or better(less) vs x2.

 

[Evo]

A more detailed shock load force analysis for locked-catching a traditional -ve rigged spar chunk is in the HSE report. Pretty sure search will root it out. I test rig confirmed their results. For practical reasons I don't think anyone has ever quantified the sledge hammer effect. How could you measure it?!

A bunch of intermediary posts just showed up for me. The same sledge hammer occurs for a locked -ve spar chunk traditional or dwt because the log chunk follows the same trajectory. The rig tip load is reduced simplistically x2 vs x1.5 dwt and the dwt rope tensions are different with mainly the down leg being about half as big as traditional -ve rig.

The big driver for the sledge hammer effect is a roughly fixed portion of the falling log's kinetic energy being direction-changed from falling trajectory direction to arcing inwards toward the stem - that's actually the source of the big 5G catch force spike. So if you let it run you take away a lot of this gnarly direction change force spike and also the sledging velocity ie smaller sledge hammer blow to the trunk. With let it run it's more of a glancing blow to the trunk.

When I used the phrase a bit less than, you can substitute in the actual rope tension change by using the appropriate tension ratio for the device you use. For a block, 1/1.2 = 0.83 tension, for an xring I think 1/1.4 = 0.71 tension and IIRC a biner 1/1.6 = 0.62 tension. I didn't want to muddy the waters for those who haven't familiarised themselves with the tension ratio principle yet.

To reiterate, the more rope in the system is almost negligible in the case of traditional -ve rig vs dwt for a hard stopped pre-locked spar chunk. The rig tip and hence spar loading is definitely less in this case for dwt vs traditional. x1 1/2 or better(less) vs x2.

The slam is real, and one the climber certainly feels. A good rope person can mitigate by getting it to slam lower (thicker) on the trunk. Or an even letting it burn cushioning the impact on the ground.
While uncomfortable I’m less concerned with the slam vs the overall triad negative rigging forces.

There are just so many variables, but I agree with all here. Unless going extreme double whippin is not worth the trouble unless it’s span rigging.